1. Field of the Invention
This invention relates to a drive apparatus for a toggle-type mold clamping mechanism of a vertical injection molding machine.
2. Description of the Related Art:
As shown in FIG. 10 of the accompanying drawings, a typical conventional drive apparatus for a toggle-type mold clamping mechanism of a vertical injection molding machine comprises a pair of toggle mechanisms, each including a pair of links 101, 101 pivotally joined at their inner ends and pivotally connected at their outer ends to a stationary platen 102 and a rear platen 103, respectively, a nut 106 to which the inner ends of the links of one toggle mechanism are connected, a ball screw on which the nut 106 is threadedly mounted, and a motor 107 to which the inner ends of the links of the other toggle mechanism are pivotally connected and which has an output shaft secured to the ball screw. When the distance between the inner ends of the links of one toggle mechanism and those of the links of the other toggle mechanism is varied, mold halves 105 secured to the bottom surface of an upper plate 104 and the top surface of the stationary platen 102, respectively, are opened or closed.
With this conventional drive apparatus for a toggle-type mold clamping mechanism, the motor is attached in an unbalanced manner to the movable parts which move vertically simultaneously with the vertical movement of the upper and rear platens. It is impossible, therefore, to keep the mold-attaching surfaces of the stationary and upper platens parallel to each other due to the weight of the movable parts and due to a deviated load on the movable parts during closing and opening of the mold halves. Further, since the weight of the movable parts becomes increased, it is time-consuming to accelerate and decelerate the motor so that the mold halves can hardly be opened and closed at high speed.